Delimiting conditions under which natural organic matter can control Fe speciation and size in freshwaters.

Pollutant and nutrient mobility in natural waters is typically controlled by sorption onto the high surface area of colloidal particles, most of which may form by precipitation of Fe(III)(hydr)oxides. Therefore, prediction of the speciation and size of Fe is critical to managing water quality. Prediction from pH and dissolved oxygen (D.O.) saturation can fail because of Fe binding to natural organic matter (N.O.M.) in natural waters. We test the influence of environmental variables -temperature, illumination and mixing order of Fe and N.O.M. with D.O.- on the impact of N.O.M.. Differences in mixing order simulate Fe(II) mixing with N.O.M. in groundwater prior to emerging, in comparison to Fe(II) emerging into oxic surface waters containing N.O.M.. Fe speciation and size were measured in waters containing N.O.M. with and without D.O., but also a water to which N.O.M. and then D.O. were added sequentially. Without D.O. free Fe(II) bound to N.O.M. and became a filterable particle. Binding increased with pH and at 7.5 was sufficient for Fe speciation in oxic waters to become influenced by whether mixing had been sequential or simultaneous. Therefore, at high pH Fe speciation in oxic surface waters requires knowledge of N.O.M. content of this water and upstream groundwaters. Cold (10 °C) decreased anoxic binding of Fe(II) to N.O.M. and both cold and darkness alsodecreased Fe binding to N.O.M. under oxic conditions, because in both cases Fe(III)(hydr)oxide surfaces out-compete N.O.M. for binding Fe. Cold and darkness therefore overwhelm the effect of mixing order on oxic Fe speciation, and cold even makes the presence or absence of N.O.M. irrelevant. In the cold or dark, prediction of Fe speciation and size in surface waters may not require knowledge of N.O.M. content of upstream groundwaters. Furthermore, when cold, prediction may not even require knowledge of N.O.M. content of the surface waters.